Lessening the carbon footprint of plastics used for consumer applications such as packaging is of increasing importance. Aliphatic polycarbonates (APCs) are recognized as an attractive option in this regard since they have a very favorable carbon profile compared to traditional polymers derived exclusively from fossil fuel feedstocks. This is due in part to the fact that a significant portion of the mass of the polymer is derived from CO2 which can be derived from waste sources. APCs not only replace a significant mass of polymer with non-fossil fuel feedstock, they can actually be produced from waste CO2 that would otherwise be released to the atmosphere. Additional factors such as lower processing temperatures and lower use of energy in production make these polymers even more favorable when compared to polymers derived exclusively from petroleum or natural gas feedstocks. Life cycle analyses of aliphatic polycarbonates also indicate they exceed bio-based polymers that require large amounts of energy and fresh water for production and in some instances compete for the same resources required for food production.
For these advantages to have a significant environmental benefit requires the identification of large markets where significant volumes of APC can replace traditional petrochemical polymers. This has been a challenge: while epoxide CO2 polymers have been known for over 40 years, they have yet to find broad application in any commodity market This is due to their relatively poor structural and thermal characteristics, and until recently, their high cost. The cost has been reduced in recent years through the identification of efficient cobalt-based catalysts for the copolymerization of CO2. Strides have also been made in improving the physical properties of the polymers. APCs made with cobalt catalysts have much better defined structures than earlier materials based on zinc or aluminum catalysts. The newer materials exhibit a very high degree of CO2 incorporation, strict control of molecular weight (Mn) and molecular weight distribution (PDI) and lower contamination by cyclic carbonate by-products.
APCs encompassing these improvements have been demonstrated to have higher glass transition temperatures, better thermal stability, and lower gas permeability. All of these improvements have increased the likelihood of adoption of the polymers in large volume consumer applications such as uses as packaging materials. However, the polymers still have some shortcomings in terms of their physical strength and flexibility. Poly(propylene carbonate) which has been the most studied epoxide-CO2 copolymer tends to be quite brittle. This is particularly true if the polymer is produced in highly pure form free of ether linkages (caused by direct enchainment of two or more epoxides without CO2) and free of cyclic propylene carbonate (cPC) (formed as a byproduct during polymerization or by partial degradation of the polymer by nucleophilic attack of the hydroxyl chain ends on adjacent carbonate linkages). In certain cases, the presence of ether linkages can lower the Tg of the polymer and provide less brittle materials, but this generally comes at the cost of strength, lower thermal stability and poorer gas barrier properties. Likewise, while residual cyclic carbonate can act as a plasticizer to make the polymer less brittle, the presence of the byproduct has undesirable side effects and may be a problem where the polymer is to be used for food contact since small molecules such as propylene carbonate can migrate from the packaging material to contaminate the contents of the package.
Attempts have been made to blend aliphatic polycarbonates with other materials to improve their applicability, but these blends have focused on biopolymers such as polylactic acid (PLA), polyhydroxybutyrate (PHB), starch and the like. These blends still suffer the environmental disadvantages of the biopolymers used in the blends and in many cases, still have only moderate processing and physical characteristics.
Polyolefins such as polyethylene (PE) and polypropylene (PP) constitute the major portion of the consumer packaging market. These polymers are popular because they provide an excellent combination of physical properties, good processing characteristics and low cost. As noted above, one area in which they lag is their carbon footprint. Blends of polyolefins with aliphatic polycarbonates are not currently known in the art.
Lower permeability to oxygen is also important in packaging applications. Good oxygen barrier properties lead to an increased shelf-life as a result of less oxidation of food and beverages, thereby maintaining taste and quality for a longer time. This is particularly important as current trends in the packaging industry are to down-gauge films by reducing their thickness to provide light-weight packaging. Thus, an improvement in permeability at an equivalent thickness or an equivalent permeability at a much lower thickness can have significant commercial value. Improved oxygen barrier films are important for packaging a variety of foods and beverages, including meat, baked goods, snacks, juices in stand-up pouches, confectionaries, and a wide variety of moisture and oxygen sensitive nutraceuticals and health and beauty products. The food packaging industry is looking for new options as they move away from current materials like polyvinylidene chloride (PVDC) due to environmental regulatory pressures on chlorinated materials and ethylene vinyl alcohol (EVOH) due to sensitivity to moisture and higher oxygen permeability at higher humidity levels.
There remains a need for APC compositions with improved physical properties. Methods to improve the properties of the APCs without sacrificing their unique environmental benefits would be particularly valuable. The present invention addresses these needs and others.